This invention relates generally to four wheel drive articulated tractors and, more particularly, to improvements to the drive system for hydrostatically driven tractors.
It is desirable to integrate all the drives for the input driven components of an articulated four wheel drive tractor into a single gearbox affixed to the rear axle housing of the tractor. Such a mechanism would place the hydrostatic pump for powering the traction drive of the tractor, the hydraulic pump for the tractor hydraulic system, the drive for the power-takeoff (PTO) system, and any auxiliary hydraulic pumps to be driven off the same gearbox device with the output driver components, including the hydrostatic motor, speed reduction gears and output drive shafts.
Lubrication of a gear set is a consideration in the design of a gearbox device for transferring rotational power through gear sets arranged and configured to provide the proper rotational speed to a component driven from the engine of the tractor. Commonizing lubrication sumps can minimize the number of lubrication systems that need to be provided as well as provide more effective lubrication of the gear sets and tile cooling of the lubrication fluid.
Conventional tractor drive mechanisms include a mechanical transmission to provide different output speeds for a given engine input speed. Even hydraulically driven tractors have utilized a three speed mechanical transmission between the hydrostatic motor and the final drive to provide desired different output speeds to the operation of the final drive for powering the movement of the tractor. It would be desirable to replace the three speed mechanical transmission on hydrostatically driven tractors to minimize cost and to enhance operation of the tractor.
Obtaining maximum drawbar pull and maximum road speed capability in a hydrostatically driven tractor requires the variable hydrostatic motor be operated at close to it's maximum rated operating speed when in road gear. Since there is little margin then remaining for overspeeding under conditions where the tractor is moving downhill, some overspeed control must be provided.
When the hydrostatic drive is used with a high torque rise engine, the available charge pump flow drops as the engine speed drops until the point of not being able to maintain servo pressure is reached, resulting in a hydrostatic stall. Under these conditions, the engine will continue to run, while the hydraulic fluid is being sent over relief creating heat.